FIG. 7 is a sectional view illustrating a prior art semiconductor laser module. In FIG. 7, reference numeral 101 designates a pedestal for supporting a semiconductor laser element and optical parts. The pedestal 101 comprises Si or the like and has a hole 107 about 100 .mu.m deep. A submount 102 comprising SiC or the like and having a thickness of 300.about.500 .mu.m is fixed on the pedestal 101. A semiconductor laser element 103 about 100 .mu.m thick is disposed on the submount 102. Wires 104 and 105 are connected to the semiconductor laser element 103 and the submount 102, respectively, and current is supplied to the laser element through these wires. A spherical lens 106 having a radius of about 300 .mu.m is set in the hole 107 of the pedestal 101. The spherical lens 106 is positioned by the hole 107. An optical fiber 109 is fixed on the pedestal 101 via a supporter 108 comprising SiC. The diameter of the optical fiber 109 is about 100 .mu.m. The optical fiber includes a core through which light is transmitted. The diameter of the core is about 10 .mu.m. The core is formed by doping a center portion of the optical fiber.
A description is given of the assembling process.
Initially, the hole 107 is formed in a prescribed position of the pedestal 101 by a conventional photolithographic technique, and the supporter 108 is formed on a prescribed part of the pedestal 101. The spherical lens 106 is set in the hole 107 and fixed using an adhesive or solder. The optical fiber 109 is fixed on the supporter 108 using an adhesive or solder. Thereafter, the submount 102 is fixed on a prescribed part of the pedestal 101 using a solder, such as Au-Sn, and a semiconductor laser element 103 is fixed on the submount 102 using a solder, such as Au-Sn. The positioning of the semiconductor laser element 103 is carried out using a marker disposed on a prescribed portion of the upper surface of the semiconductor laser element 103 opposite the laser light emitting point. Preferably, the marker is a metal film. Thereafter, the wires 104 and 105 are connected to the semiconductor laser element 103 and the submount 102, respectively, preferably by ultrasonic heating. The other ends of the wires 104 and 105 are connected to an external terminal of a voltage supply (not shown).
A description is given of the operation. When a voltage is applied across the wire 104 connected to an external terminal of the semiconductor laser element 103 and the wire 105 connected to the submount 105, current flows through the semiconductor laser element 103, and laser light is emitted from a light emitting point of the semiconductor laser element 103. The emitted laser light is collected by the lens 106 and applied to the center of the facet of the optical fiber 109, i.e., the core of the optical fiber 109.
In the present optical fiber communication, it is possible to transmit optical signals at a rate exceeding 2.5 G bit/sec by high-speed on-off switching of the voltage applied across the wires 104 and 105 to change the light intensity, i.e., by direct modulation.
In the prior art semiconductor laser module, in order to utilize the emitted laser light from the laser element 103 with high efficiency, the positions of the semiconductor laser element 103, the spherical lens 106, and the optical fiber 109 must be precisely determined so that the laser light emitted from the laser element 103 and collected by the spherical lens 109 is applied to the core of the optical fiber 109 with high reliability. Therefore, in the prior art laser module, the semiconductor laser element 103, the lens 106, and the optical fiber 109 are positioned on the pedestal 101 so that the light emitting point of the semiconductor laser element 103, the center of the lens 106, and the core of the optical fiber 109 are arranged in a straight line at prescribed intervals.
However, since the diameter of the core of the optical fiber 109, i.e., the effective diameter for transmitting laser light, is only 10 .mu.m, if the precision of the assembly of the laser module is poor, it is difficult to apply the laser light emitted from the laser element 103 to the optical fiber 109. In the prior art semiconductor laser module, when the semiconductor laser element 103 is mounted on the submount 102, the position of the laser element 103 is observed from above so that the light emitting point of the semiconductor laser element 103 is disposed on a prescribed position of the pedestal 101, whereby the positioning accuracy of the semiconductor laser element 103 in the prescribed direction on the surface of the pedestal 101 is improved. However, the position of the light emitting point of the semiconductor laser element 103 in the direction perpendicular to the surface of the pedestal 101 depends on the thickness of the submount 102, the thickness of the solder connecting the submount 102 and the pedestal 101, the thickness of the semiconductor laser element 103, and the thickness of the solder connecting the submount 102 and the semiconductor laser element 103. Usually, in order to apply the emitted laser light to the core of the optical fiber 109 with high reliability, the positioning error of the light emitting point of the semiconductor laser element 103 in the direction perpendicular to the surface of the pedestal 101 must be lower than about 10 .mu.m. However, since the total of the errors in the thickness of the submount 102, the thickness of the solder connecting the submount 102 and the pedestal 101, the thickness of the semiconductor laser element 103, and the thickness of the solder connecting the submount 102 and the semiconductor laser element 103 exceeds 10 .mu.m, the positioning error of the light emitting point exceeds 10 .mu.m. Because of the insufficient positioning precision, the laser light is not reliably applied to the core of the optical fiber 109, resulting in poor performance of the semiconductor laser and variations in the performances of each of a plurality of semiconductor laser modules.
Further, in this prior art laser module, since solder providing poor thickness controllability is present at two boundaries between the submount 102 and the pedestal 101 and between the submount 102 and the semiconductor laser element 103 in the direction perpendicular to the surface of the pedestal 101, the positioning precision of the light emitting point of the laser element cannot be improved.
Although it is possible to improve the processing precision of the thicknesses of the submount 102 and the semiconductor laser element 103 in order to solve the above-described problems, the improved processing precision is adverse to mass production, resulting in an increase in the production cost.
Meanwhile, there is another assembling method of the semiconductor laser module shown in FIG. 7. In this method, initially, the submount 102 with the semiconductor laser chip 103 is fixed on the pedestal 101 having the positioning hole 107 and the supporter 108, and the wires 104 and 105 are connected to the semiconductor laser element 103 and the submount 102, respectively, preferably by ultrasonic heating. Thereafter, the lens 106 is set in the hole 107 of the pedestal 101 and the optical fiber 109 is fixed on the supporter 108. In this assembling method, the intensity of light incident on the optical fiber 109 is measured while current flows through the wires 104 and 105, and the optical fiber 109 is fixed to the pedestal 101 when the intensity attains a maximum, whereby the laser light is sufficiently applied to the core of the optical fiber 109 without improving the processing precision of the submount 102 and the semiconductor laser element 103. As the result, a semiconductor laser module with sufficient performance is obtained. In this prior art method, however, the positioning of the optical fiber 109 takes about 10 minutes, so that the productivity is reduced and the cost of the assembly is increased, resulting in an expensive semiconductor laser module. In addition, since the laser light incident on the optical fiber 109 has a plurality of peaks because of scattering of the incident light, it is difficult to find a true peak of the emitted laser light.